The present invention relates generally to open cycle vapor compression heat pumps and, in particular, to the simultaneous and independent control of liquid level and vapor pressure in the flash tank of an industrial process heat pump.
An open cycle vapor compression heat pump useful for economically recovering heat content of a liquid typically utilizes as an input thereto a waste liquid warmed by processes or sources such as condensers cooling towers and compressor cooling which produce fluctuations in the flow rate and in the temperature of this liquid. As a result, the liquid level and fluid pressure in the flash tank portion of the heat pump are also subject to variation. Unless the liquid in the tank is maintained at an appropriate level for the tank, damage to other components in the heat pump system is likely to occur. For example, if the fluctuations in the flow rate of the warm liquid entering the system cause a large amount of liquid to enter the tank, the liquid will fill the tank and enter the compressor, resulting in compressor failure. Conversely, if little or no warm liquid enters the heat pump, the flask tank liquid level will drop below the acceptable level, or, depending on the fluctuations in flow rate of the entering liquid, the tank may empty completely. In this case vapor from the tank will enter the discharge pump, causing it to cavitate and eventually to fail. Failure of either the compressor or the discharge pump renders the entire system inoperable and is likely to require replacement of these potentially costly components.
However, not only must the flask tank liquid level be kept at an appropriate level to prevent the problems just discussed, but the tank fluid pressure must also be maintained at the design level for the heat pump to assure efficient functioning of the system compressor. If the fluid pressure in the flash tank is not monitored and regulated so that it is maintained at the design level, the compressor discharge pressure will be affected.
For a typical compressor such as a centrifugal compressor which is operated at a constant speed, the ratio of the outlet pressure to the inlet pressure is relatively constant. Thus the pressure to which the vapor from the flash tank is elevated by the compressor will decrease as the inlet vapor pressure decreases causing a loss in steam quality at the output of the compressor. Further, the thermal efficiency of the compressor is proportional to T.sub.H /(T.sub.H -T.sub.L) where T.sub.H and T.sub.L are the saturation temperatures at the outlet and inlet, respectively, of the compressor. Since the warm liquid at the input of the flash tank is flashed as it enters the tank, the liquid and vapor contained therein are saturated. Thus if the pressure of vapor in the flash tank decreases, its temperature will also decrease causing an undesirable decrease in thermal efficiency of the compressor. In addition, compressor power requirements are affected by the tank fluid pressure because of the interrelationship of tank fluid pressure and density of the vapor at the compressor inlet. For example, excessive inlet vapor density due to tank fluid pressure in excess of the design pressure will cause increased power demands on the compressor. If, as would be the situation when the tank fluid pressure is not controlled, compressor power in excess of the design rating is continually required, compressor failure will eventually result.
Efforts to control only the level of liquid in the tank, while helpful in averting compressor and discharge pump failures, have not achieved the efficiency of operation desired. Consequently, the level of the liquid and pressure of the fluid in an open cycle vapor compression heat pump must be carefully controlled simultaneously to assure efficient operation of the heat pump system and to avoid failure of the heat pump components. There is a need, moreover, for an arrangement of controls for an open cycle vapor compression heat pump which simultaneously and independently monitors and regulates both the level of liquid and the pressure within the system flash tank so that the heat pump functions at optimal efficiency and is not likely to be subject to costly equipment failures.
It is, therefore, an object of the present invention to control simultaneously and independently both the level of liquid and fluid pressure in a heat pump flash tank to achieve efficient operation of the heat pump and to prevent failure of the compressor and/or discharge pump.
It is another object of the present invention to provide in an open cycle vapor compression heat pump to regulate the level of liquid in the flash tank and to maintain tank pressure at the design level.
It is a further object of the present invention to provide an open cycle vapor compression heat pump having an inlet throttle valve which operates in response to fluid pressure in the flash tank and a pump discharge throttle valve which operates simultaneously with and independently from the inlet throttle valve in response to the level of liquid in the flash tank.